Abstract: A label-attaching machine for attaching labels carried on a partially transparent dispensing web for attachment to serially presented envelopes has a two-beam optical sensing unit to govern the synchronization of label feeding by determining the location of the leading edge of the next label to be dispensed. One beam senses the optical transmission of label-bearing regions of the web, and the other beam reads the optical transmission of label-free used web. A comparator circuit balances the signal strengths of the two beams to provide a signal condition indicative of label edge passage. A zip code sort is provided for use in conjunction with the same optical sensor. Labels of a common zip code are serially affixed to the web, and the arrival of a new zip code is flagged by an oversize gap between the last label bearing one zip code and the next label bearing the first of the new zip codes.